A pan-tilt-zoom (PTZ) camera can acquire a high angular resolution image or video of a small portion of a hemispherical scene by setting the field of view to a narrow angle. However, while the camera is directed on the small portion of the scene, the remaining portion of the scene cannot be viewed. Several solutions to this problem are known.
Temporal Multiplexing
In temporal multiplexing, the field of view of the PZT camera is normally set to a very wide angle and short focal length to acquire low detail, wide field of view images. When more detail is required, the PZT camera is directed at a particular portion of the scene to acquire narrow field of view images. Typically, this is done manually. For example, a user locates a significant surveillance event in the scene from the wide field of view images. The event can be a moving object, such as a person, car, door, or a other change in the environment, e.g., smoke or fire. Then, the user manually directs the camera at the event to acquire more detailed images. As an advantage, all images are acquired by a single camera. However, as a disadvantage, the wide angle and detailed images cannot be acquired at the same time, which may cause some significant events to go undetected.
Distributed Sensors
Multiple sensors can be distributed in an environment along with a single PTZ camera. For example, the sensors can be fixed cameras or motion detectors. In this arrangement, the sensors detect events and the PZT camera is directed at the events. For example, if a sensor detects the opening of a door, then the PTZ camera can be directed at the door. As an advantage, events can still be detected while the PZT camera is directed elsewhere. However, for this type of system to operate, the PZT camera and the sensors must be calibrated so that each sensor is mapped to a particular geometric orientation of the PTZ camera. This problem is repeated every time the configuration of the environment changes. If the system is operated manually, it may be difficult to direct the PZT camera at events in a timely manner, due to the perceptual gap between the sensor network observations, and the PTZ control space.
Multiple Cameras
It is also possible to use a wide-angle camera in conjunction with a PZT camera. This arrangement also requires calibration, particularly when the wide angle camera and the PZT camera are manufactured as separate units. If the fields of view of the two cameras have some overlap, then manual operation of the system is relatively easy. As a disadvantage, extremely wide-angle refractive lenses are expensive, and such lenses cause significant non-linear distortions, which make it difficult to calibrate the system for automatic operation. However, the worst aspect of this arrangement is that cameras with refractive lenses are a bad match for PTZ cameras. For example, if the PTZ camera is placed in a corner, then the camera can only view about one eighth of the view sphere and a moderately wide-angle refractive lens is sufficient. However, most PTZ cameras are capable of viewing at least a hemisphere, and many can view far more than that. Covering such a wide field of view with a single refractive lens is not possible. Adding cameras increases the calibration cost and the likelihood of the perceptual gap problem described above.
Virtual PTZ
One could construct a virtual PTZ camera by combining a single, high-resolution sensor with wide-angle optics. The system can then decode both wide-angle frames, as well as high-resolution detailed frames from a sequence of images. However, this is not generally practical.
For example, the Sony SNC-RZ30N camera has a 1-25× zoom lens. At a widest setting, the camera has a 25° horizontal field of view. This means that each pixel represents roughly 4.6×10−7 steradians of the view sphere. Furthermore, the camera can observe about 3π steradians of the view sphere. Therefore, a single-sensor camera requires at least 20×106 pixels to replicate the acuity of the wide-angle setting of the PTZ camera. For the narrow-field, the camera has over 25 times the angular resolution, so a single image sensor needs at least 252 more pixels, or 13×109 pixels, which is about a thousand times the resolution of currently available cameras. Even if a gigapixel sensor could be manufactured, the cost of communicating all those pixels as a video stream would be prohibitive.